1. Field of the Invention
The present invention relates generally to optical sensors and more particularly to windows for protecting optical sensors in hazardous environments.
2. Description of the Related Art
Optical sensors are used for in situ or on-line monitoring of applications in hazardous environments because of they are nonintrusive and can be highly sensitive. Combustion diagnosis using silicon carbide (SIC) flame sensors has been disclosed in commonly assigned Brown et al., U.S. Pat. No. 5,257,496, for example. Combustion flame temperature and dynamics of gas and oil fired turbines have been measured by high temperature SiC flame sensors. The signals produced by the optical sensors are well-correlated to conventional measurement devices such as thermocouples for temperature, mass spectrometers for nitrogen oxides (NOx), and pressure sensors for dynamics. SiC optical sensors are advantageous because they have the potential for highly reliable remote sensing of the optical emissions from the combustion process whereas direct contact sensors such as the conventional measurement devices sometimes become damaged by harsh conditions in the combustors.
An optical sensor can have a window shielding it from the sensed environment. One difficulty challenging flame sensor and optical sensing applications is that the window can become coated with combustion soot. The transmission of the window directly affects the light received by the photodetector, causing calibration change and loss of signal.
Another technical difficulty is the high temperature limit on the integrity of hermetic bonding between optical translucent window material for ultraviolet, visible, or infrared light, and the sensor housing which typically comprises metal. Conventional brazing technology for sapphire or quartz with metal guarantees up to 400.degree. C. or lower for high differential pressures such as in gas turbines where 250 pounds per square in inch can be normal. The temperature of 400.degree. C., however, is not hot enough to induce self-burning of all carbon deposits. Therefore, it is not possible to keep the hermetically sealed window from soot deposit for flame sensors next to the combustors.